Question

Oxidation state of oxygen in $Cr{O_5}$is
(A) -1
(B) -2
(C) Both (A) and (B)
(D) $ - \dfrac{1}{2}$

Answer 1

Hint: Oxidation state of any atom indicates the degree of oxidation for an atom in a chemical compound. It is the hypothetical charge that an atom would have if all bonds of the atoms of different elements were completely ionic. In order to find the oxidation state of $Cr{O_5}$, we should know its structure.

Complete step by step answer:
Chromium peroxide also known as chromium oxide peroxide is an unstable compound with the formula $Cr{O_5}$. This compound contains one oxo ligand and two per oxo ligands, making a total of five oxygen atoms per chromium atom in its structure. Chromium peroxides are formed by the addition of acidified hydrogen peroxide solutions to the solutions of sodium chromate or potassium dichromate. They are generally yellow chromates or orange dichromats which eventually turn to dark blue as chromium (VI) peroxide is formed. Chromate or dichromate reacts with hydrogen peroxide and with an acid to give chromium peroxide and water.
$Cr{O^{ - 2}}_4 + 2{H_2}{O_2} + 2{H^ + } \to Cr{O_5} + 3{H_2}O$
With this method, the chromium (VI) peroxide will decompose after a few seconds, turning green as chromium (III) is formed.
$2Cr{O_5} + 7{H_2}{O_2} + 6{H^ + } \to 2C{r^{ + 3}} + 10{H_2}O + 7{O_2}$
To avoid this decomposition, it is possible to stabilize chromium(VI) peroxide in water-immiscible organic solvents such as diethyl ether or butan-1-ol by adding a layer of the organic solvent above the chromate or dichromate solution and shaking during the addition of hydrogen peroxide. In this way, the chromium (VI) peroxide gets dissolved in the immiscible organic solvent. In this condition, it can be observed over a much longer period.
Since Cr has five electrons in 3d orbitals and one electron in 4s orbital. This value is since four oxygen atoms in $Cr{O_5}$ are in peroxide linkage. In $Cr{O_5}$there is one oxygen double bonded to the chromium atom and there are two peroxide linkages (-O-O-). This will result in a +6 oxidation state of $Cr{O_5}$. One oxygen is double bonded with Cr atom which results in its -2-oxidation state and due to the presence of two peroxides, both of their oxidation states are -1.

Therefore, the correct answer to the above question is (C)- Both (A) and (B).

Note: It is a point to remember that $Cr{O_5}$ is a good oxidizing agent, even better than chromates and dichromates due to the presence of two highly reactive per oxo ligands. But since it is not a stable compound and eventually breaks into $C{r^{ + 3}}$, it is not used in organic syntheses.